Upon recording of data from a music or image source on an optical disc recording medium, the data are converted into a digital signal having logic "0" and "1" levels. According to the logic level, pits are formed to provide a continuous spiral recording track in the optical disc recording medium. An optical pickup device is used to reproduce the recorded data by radiating a light beam from a light source such as a semiconductor laser through an objective lens which focuses the light beam so as to produce a light spot of a predetermined constant diameter on the optical disc recording medium and sensing the light beam reflected from the optical recording medium to determine the presence of pits. For this purpose, the objective lens is supported in such a manner that its optical axis can follow the spiral recording track while the optical disc recording medium is rotating during the data reproducing operation.
A grating technique is used to divide the light beam in a manner to produce three light spots on the optical disc recording medium. These light spots include a main spot 2A and two side spots 2B and 2C produced on the opposite sides of the main spot 2A and offset a predetermined distance in a direction normal to the diameter of the optical disc recording medium, as shown in FIG. 6. The light beam reflected on the main spot 2A is used for data reproduction and also focusing servo control. The light beams reflected on the side spots 2B and 2C are used for tracking servo control.
In order to detect the pits with high accuracy, the light spots 2A, 2B and 2C should be retained in position with respect to the recording track 1a formed in the optical disc recording medium 1. If the orbit L of movement of the objective lens (not shown) is parallel with and spaced a distance .DELTA.r away from a straight line T extending through the center O of rotation of the spindle, the main spot 2A will deviate from a desired recording track and the tracking error signal resulting from the light beams reflected on the side spots 2B and 2C will have a reduced modulation factor. Assuming now that .theta. is the angle made by the straight line extending through the side spots 2B and 2C with respect to the tangent of the recording track 1a at a point through which the straight line T extends, .theta..sub.1 is the angle made by the straight line extending through the side spots 2B and 2C with respect to the tangent of the recording track 1a at a point through which the orbit L extends, and R is the distance between the recording track 1a and the center O of rotation of the spindle, the following equation is established: EQU .theta..sub.1 =.theta..+-.(.DELTA.r/R) 1)
As can be seen from Equation (1), the deviated angle .DELTA..theta. (=.theta..sub.1 -.theta.) resulting from the offset .DELTA.r is dependent on the radius of the optical disc recording medium 1. This is referred to as "radial dependence".
It has been proposed, in Japanese Patent Kokai No. 63-71934 entitled "Optical Pickup Driving Apparatus", to avoid the difficulty resulting from the radial dependence by rotatably supporting the feed shaft, which is used to guide the optical pickup device, in a manner to adjust the orbit of movement of the objective lens into coincidence with the straight line extending through the center of rotation of the spindle. With such a conventional optical pickup driving apparatus arranged to rotate the feed shaft upon adjustment of the deviation of the light spots, however, the rotation of its optical system is limited within a narrow range. As a result, the amount by which the deviation of the light spots can be adjusted is limited and the capacity of absorbing errors occurring upon attaching the optical pickup device is limited.